The prior art foresees examples of devices or actual underwater robots, with pulsed jet propulsion and inspired by biomimetics. Amongst these, the device described in patent publications U.S. Pat. No. 3,154,043 and CN201712781, and the craft described in Yangwei Wang, et al.: Novel design for a biomimetic water-jetting propulsion vehicle actuated by SMA wires, Applied Mechanics and Materials, vol. 50-51, 2011, pp.73-77. The latter in particular replicates the propulsion system of a squid, foreseeing a semi-cylindrical mantle made from flexible material such as silicone gel, defining, in diametrical cooperation with a rigid shell, a cavity with an inlet opening and an outlet nozzle of the liquid in the axial direction. A framework of semi-rigid supports embeds the mantle along respective generatrices, and therefore in an axial/longitudinal direction.
The contraction and the expansion of the mantle, resulting in the ejection of pulsed jets for the propulsion from the aforementioned nozzle, is managed by wires made of SMA (Shape Memory Alloy) which extend over the mantle between the semi-rigid supports in a circumferential direction. The activation of the SMA wires by means of electric heating induces a contraction thereof and thus a movement of the supports that comes together with a consequent reduction of the inner volume of the mantle. The deactivation of the wires allows, on the other hand, the mantle to expand again, drawing the fluid inside the cavity through the inlet opening. The rigid diametrical shell, which also represents a structural element of the craft, houses the system for controlling and electrically urging the SMA wires.
The invasive presence of rigid or semi-rigid components is an element that strongly limits the craft/device described above, thus leading to the possibility of damaging the device itself and of the surrounding environment, and to a lower intrinsic safety that deters its use and operation in the vicinity of people and animals. It moreover penalises the manoeuvrability, making the structure heavier and less hydrodynamic. At the expense of the manoeuvrability there is also the SMA wire actuation system, which is abrupt and thus makes the movement markedly discontinuous and more difficult to control.